The invention relates generally to semiconductor fabrication and more specifically to in-situ film thickness measurement for thin films deposited on a semiconductor substrate utilizing broad band spectrometry.
During semiconductor fabrication there are multiple steps where an underlying substrate is subjected to the formation and removal of various layers. The small feature sizes and tight surface planarity requirements, combined with the constant quest to increase throughput, makes it highly desirable to stop the process when the correct thickness has been achieved, i.e., when an endpoint has been obtained for the process step.
Present optical end point detection (EPD) methods make use of Broad Band Visible Spectra Spectrometry. The light of a lamp flash, i.e., shot, is returned from a wafer while an upper film is removed through a chemical mechanical planarization (CMP) or etch process. The light spectrum returned is measured for each shot and analyzed according to a programmed algorithm. For many multilayer transparent thin film structures, such as films associated with shallow trench isolation (STI) applications, the reflectance from the silicon substrate, is much greater then from upper layer film interfaces. Accordingly, the underlying silicon provides the main contribution in observed reflectance spectra. For instance, for a STI structure the reflectance from the silicon (Si) substrate surface, created at prior technological step trenches provides the main contribution in total reflectance. As a result, the reflectance spectra changes related to the layer of interest, i.e., the upper layer, are relatively small from shot to shot even at the moment when upper layer is fully removed. Therefore, EPD methods based on detecting the transition from one layer to the next underlying layer are not efficient enough when dealing with these thin transparent films encountered during upper layer removal processes. In such cases the method of direct measurement of the film thickness based on broadband visible spectrometry is preferred. For transparent films the measured spectra are the result of complex interference of light reflected from the wafer surface and deeper layers of the wafer, and the light scattered back from interlayer material. To obtain film thickness from these complicated spectra, Fourier transform and simulation spectra methods are typically used. One skilled in the art will appreciate that Fourier transform expends the complex spectrum on periodic terms. Peak locations in Fourier transform spectrum allow to restore film thickness of the layers in a stack. This method is applicable to relatively thick films. The periodic term corresponding to the film thickness should have enough repetitions in the measured spectrum so that peaks will be well separated and distinguished. However, when the spectrometer measurement band is 300 nanometers-700 nanometers, this method is inadequate for measuring the thickness of a layer that is less then about 300-600 of a nanometer (nm).
The simulation spectra methods are based on a multiparametric model where the thickness of layers, reflectance of interlayer boundaries and other characteristics of the structure taken as parameters. The multiparametric model disadvantages include high sensitivity of the approximation to precise values of introduced parameters and existence of numeral, very close fittings with completely different sets of fitting parameters. This uncertainty, increases with the number of terms and parameters in the fitting model. In addition, the above described methods are ineffective for layer thickness measurement if reflectance of the layer surfaces is only small part of total reflectance, such as the upper layer in an STI structure. That is, the thickness of the layer can not be extracted from such a spectrum because of the dominance of the reflectance from silicon and multiple unknown parameters.
In view of the foregoing, there is a need to provide a method and system to measure the thickness of layers deposited on a semiconductor substrate through broadband visible spectrometry irrespective of the contribution of the reflectance of the layer surface to the total reflectance.